1. Field of the Invention
The present invention relates to a flexible magnetic disc. More particularly, it relates to a flexible magnetic disc with good surface lubricity, improved durability and decreased drop out as well as good electrical properties.
2. Discussion of Prior Art
In order to increase resolution of a magnetic disc in high density recording at a short wavelength, various losses such as spacing loss, loss due to a medium thickness and self-demagnetization loss should be decreased. These losses are preferably decreased by making the magnetic layer of the magnetic disc very thin. Therefore, the thickness of the magnetic layer is usually made as thin as 1 .mu.m. When the thickness of the magnetic layer is, however, made so thin, several problems occur:
(1) When the thickness of the magnetic layer is as thin as 1 .mu.m, the cohesive force of the magnetic layer itself is decreased. In addition, since an absolute amount of a lubricant which imparts lubricity on the surface of the magnetic disc in the magnetic layer is decreased, the durability of the magnetic disc deteriorates;
(2) When thickness of the magnetic layer is as thin as 1 .mu.m, the magnetic layer tends to be affected by properties of a surface of a non-magnetic flexible substrate. That is, characteristics of the magnetic layer are directly influenced by minute roughness, flaws or fish eyes on the surface of the substrate and/or fillers or oligomers contained in the substrate. This causes minute fluctuation of output or drop out;
(3) Surface electric resistance of the magnetic layer greatly varies with the thickness of the layer and exponentially increases as the thickness of the layer decreases. Therefore, when the thickness of the magnetic layer is 1 .mu.m, the surface resistance becomes rather large so that the layer tends to be electrified. Thus, discharge between the surface of the magnetic disc and a magnetic head causes electrostatic noise, which is one source of error.
Various proposals have been made to solve these problems. Now, such proposals will be discussed. For example, to mainly solve the above described problem (1), it has been proposed to add a hard solid additive, such as Al.sub.2 O.sub.3 or Cr.sub.2 O.sub.3. Although durability of the magnetic disc is improved by this technique, the above described problems (2) and (3) are not solved. In addition, the solid additive may abrade the magnetic head. On the other hand, it is known that the problems (2) and (3) are solved by increasing smoothness of the surface of the non-magnetic flexible substrate or by reducing direct effect of the surface properties of the substrate to the magnetic layer. This is achieved by coating the surface of the substrate with an undercoat comprising a binder made of a polyester or polyurethane resin and optionally a cross linking agent or a resin having radiation sensitive functional groups, or an undercoat comprising a binder and electrically conductive fine powder which reduces electric resistance, being dispersed in the binder having a thickness of about 0.5 .mu.m to about 5.0 .mu.m and applying the magnetic layer on the undercoat. In addition, the undercoat improves adhesiveness between the magnetic layer and the substrate. However, this technique does not improve the durability of the magnetic disc and does not solve problem (1).
When the durability of the magnetic disc is discussed, two categories of durability should be investigated, one of which relates to continuous traveling (hereinafter referred to as "traveling durability") and another of which is against impact force generated when the magnetic head catches the magnetic disc like a clip (hereinafter referred to as "tap durability"). When the thickness of the magnetic layer is about 2.5 .mu.m, good traveling durability is achieved by applying and impregnating a suitable amount of a lubricant on the surface of the magnetic disc after the application of the magnetic coating and/or by adding the lubricant to the magnetic coating composition. However, when the magnetic layer is as thin as 1 .mu.m, the cohesive force of the magnetic layer itself is reduced, and the absolute amount of the lubricant impregnated or present in the magnetic layer so small that it cannot improve the durability. If the lubricant is forced to be impregnated in or added to the magnetic layer in a large amount, the binder is plasticized by the lubricant so that the mechanical properties of the magnetic layer and, in turn, the durability of the magnetic disc may deteriorate. In this case, a large amount of the lubricant bleeds out on the surface of the magnetic layer so that the surface becomes sticky and foreign particles easily adhere to the surface. Further, the magnetic head and the magnetic disc stick to or absorb each other. As to tap durability, when the magnetic disc comprises a flexible substrate as thick as about several tens of .mu.ms and a magnetic layer as thin as 1 .mu.m, the magnetic disc has a poorer ability to relax the impact force due to tap than a magnetic disc having a comparatively thick magnetic layer, and brittle fracture due to fatigue of the magnetic layer tends to result, which may result in drop out. To improve the durability of the magnetic disc, many attempts have been made including the addition of a solid additive to the magnetic layer as described above, the use of a new material which can impart lubricity to the magnetic layer, improvement of the binder and selection of a new magnetic material and/or reinforcement. However, these attempts have not satisfactorily solved the problems.